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QRS Complex

Understanding the EKG Signal

By Steve S. Ryan, PhD

An electrocardiogram, ECG (EKG), is a test used to measure the rate and regularity of heartbeats, as well as the size and position of the chambers, the presence of any damage to the heart, and the effects of drugs or devices used to regulate the heart.

The ECG signal strip is a graphic tracing of the electrical activity of the heart. It measures the length of time it takes for the initial impulse to fire at the Sinus Node and then ends in the contracting of the Ventricles.

Schematic diagram of normal sinus rhythm for a human heart as seen on ECG; Public Domain image;

Schematic diagram of normal sinus rhythm for a human heart as seen on ECG

The first upward pulse of the EKG signal, the P wave, is formed when the atria (the two upper chambers of the heart) contract to pump blood into the ventricles. In A-Fib you will see many “fibrillation” beats instead of one P wave.

The next large upward spike segment, the QRS Complex, is formed when the ventricles (the two lower chambers of the heart) are contracting to pump out blood.

The next section, the ST segment, measures the end of the contraction of the ventricles to the beginning of the rest period before the ventricles begin to contract for the next beat.

The next slight rising section, the T wave, measures the resting period of the ventricles.

ECG (EKG) Strip: Atrial Fibrillation

In the case of Atrial Fibrillation, the consistent P waves are replaced by fibrillatory waves, which vary in amplitude, shape, and timing (compare the two illustrations below).

ECG tracing of normal heart rhythm and heart in A-Fib; Copyright 2012 A-Fib, Inc.

© 2012 A-Fib, Inc.

ECG recorder: special graph Paper

The output of an ECG recorder is a graph (or sometimes several graphs, representing each of the leads) with time represented on the x-axis and voltage represented on the y-axis. A dedicated ECG machine would usually print onto graph paper which has a background pattern of 1mm squares (often in red or green), with bold divisions every 5 mm in both vertical and horizontal directions.

Diagram of electrocardiogram paper; Public Domain image

Diagram of electrocardiogram paper.

Interpreting a ECG strip involves counting the squares of the tracing. For example, by counting the squares of a heart in Normal Sinus Rhythm, you can calculate the heart rate.

To learn more about reading an ECG strip, see an excellent handout from the St. Petersburg College, School of Nursing, ‘EKG Interpretation: The Easy Rs‘.

References & Photo Credits for this article

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Last updated: Thursday, August 27, 2015 

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